Computerized x-ray tomography produces images of internal body organs which are free from the shadow of intervening structures. Prior art tomographic equipment, has generally, comprised an x-ray source disposed opposite one or more x-ray detectors on a movable structure. The source and detectors rotate and/or translate in a plane through the body organs undergoing examination to produce electrical signals, representative of views along a plurality of ray paths. The signals are then combined, usually in digital computer equipment, to reconstruct shadow-free images of internal body sections. Tomography equipment of this type is described, for example, in U.S. Pat. No. 3,778,614 to Hounsfield.
The rate of production of images in a tomography system which incorporates moving sources and detectors is necessarily limited by the time required to accomplish the physical translation or rotation of the mechanism and is, typically, limited to less than one image per second. Such equipment is, therefore, unsuited for producing moving pictures of body organs, for example, of a beating heart. Dr. Earl Wood of the Mayo Clinic has recently proposed a tomographic system for imaging moving body organs wherein a plurality of x-ray sources are sequentially pulsed to rapidly produce x-ray transmission data along a number of diverse ray paths.
The x-ray detectors utilized in prior art x-ray tomography apparatus have generally comprised scintillation crystals or phosphor screens coupled to optical detectors, for example, image orthicon or photomultiplier tubes. Such devices are rather large and must, generally, be utilized with collimation apparatus to achieve fine spatial resolution. Such scintillation detectors and collimation apparatus are, relatively inefficient detectors of x-ray energy. It is, therefore, necessary to expose a patient undergoing tomographic examination in such equipment to a relatively high dose of ionizing radiation.
My copending patent application Ser. No. 616,930, filed Sept. 26, 1975, with Nathan R. Whetten describes a high pressure, xenon filled ionization chamber array which is characterized by high detection efficiency and fine spatial resolution when utilized in x-ray tomography equipment. The detector comprises a large plurality of detector cells separated by substantially parallel metal collector plates which may be focused on a single source of diverging x-rays. X-ray photons entering the detector cells produce ion-electron pairs which drift under the influence of an electric field, in a direction parallel to the line of the detector array and substantially perpendicular to the direction of the incident x-ray beam, to the collection plates. Detectors of this type are well suited for the efficient detection of diverging x-ray energy which, for example, may be produced from a single x-ray source and collimated to provide a planar, fan-like spatial distribution. The ion chamber array of that disclosure is, however, relatively inefficient for detecting x-ray energy which originates from an array of spatially separated x-ray sources of the type utilized in the above-described, high speed tomographic equipment.